Microsoft explains WMF decision

Following speculation that a widely publicised vulnerability in Microsoft's Windows operating system may have been created deliberately, an executive within the company's security group has published a detailed explanation of how the vulnerability was introduced.

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Following speculation that a widely publicised vulnerability in Microsoft's Windows OS (operating system) may have been created deliberately, an executive within Microsoft's security group has published a detailed explanation of how the vulnerability was introduced.

His conclusion: the WMF (Windows Metafile) security bug, which first came to light last month, was not an intentional 'back door' added to give Microsoft a secret way of gaining access to its customers' PCs.

"There's been some speculation that this trigger was somehow intentional," wrote Stephen Toulouse, security program manager at Microsoft's security response center. "That speculation is wrong."

Toulouse's comment, posted at the weekend, came after security researcher Steve Gibson speculated that Microsoft had intentionally included a known vulnerability in a graphics rendering component of its OS.

"The only conclusion that can reasonably be drawn is that this was a deliberate back door put into all of Microsoft's recent editions of Windows," wrote Gibson, the founder and president of Gibson Research, in a posting to his website last week. "Why it was put in and who knew about it, and what they were expected to use it for... we'll never know."

Toulouse said that Microsoft has been fielding customer questions on this topic, many of which he assumed to have been triggered by Gibson's post. "We had been looking into detailing the history anyway, and some customer questions drove the idea to write it up," he wrote in an email interview. "We just wanted to make sure people had the history."

The vulnerability in question concerns the way that Windows processes WMF graphics files, which are used by computer-aided design programs. In the 1990s, Microsoft added a function to Windows, called SetAbortProc, that is used in processing these files. Because of a design error in the function, it can be used by hackers to take control of a Windows computer, Microsoft says. It fixed the error in a Windows patch, entitled MS06-001, released earlier this month.

Gibson had previously argued that because the SetAbortProc function could not be triggered by a correctly formed WMF file, it served no legitimate purpose, a claim that Toulouse disputed in his posting. In an interview yesterday, Gibson conceded that he had been in error on that point. "I was wrong about this," he said. "It is more complex than that, exactly as [Toulouse] explained in his posting."

Still, Gibson stood by his previous conclusion, arguing that Microsoft appeared to have intentionally changed the SetAbortProc function around the time of Windows NT to make systems vulnerable to the coding error. "The best way to characterise this is, it's intentionally designed code that, without question, enables back-door functionality," he said.

Toulouse declined to comment on this claim, but in his blog posting, he wrote that it is more difficult to exploit the vulnerability in the Windows 95 and Windows 98 OSs.

Some security researchers were sceptical about Gibson's theory. If Microsoft truly wanted to add a back door to Windows, there are better ways the company could have done it, said Cesar Cerrudo, CEO of security research firm Argeniss in Parana, Argentina. "I don't think Microsoft would use that kind of back door," he said via instant message. "It doesn't need to do that. It could just build an exploit for some unfixed remote vulnerability on Windows."

Gibson said that while he has no proof of Microsoft's motives, he believes that such a back door could have been created without malicious intent – perhaps as a way for the firm to provide assistance to users, for example. "It would be a way for Microsoft to help people who had shut their computers down, security-wise," he said.